Caption: Lichen on ash bark. Close-up of the bark of a common ash tree (Fraxinus excelsior), showing the lichen community growing on it. The bark of the ash tree is both smoother and of a higher pH (less acidic) than most other common forest trees. This encourages the growth of small crustose lichens. The high pH of the bark also buffers the surface from acidifying gases such as sulphur dioxide and nitrogen oxides, major components of air pollution. Species present here include Arthonia (black apothecia) and Lecanora (pale orange apothecia). A knot in the bark is at top. Photographed in Cumbria, UK.